lib/printk: Use picolibc's vfprintf instead of cbvprintf

When using picolibc, change vprintk to use picolibc's vfprintf instead of
cbvprintf.

Building samples/hello-world for qemu-cortex-m3.

old:

Memory region         Used Size  Region Size  %age Used
           FLASH:        9056 B       256 KB      3.45%
            SRAM:        3960 B        64 KB      6.04%
        IDT_LIST:          0 GB         2 KB      0.00%

new:

Memory region         Used Size  Region Size  %age Used
           FLASH:        7900 B       256 KB      3.01%
            SRAM:        3960 B        64 KB      6.04%
        IDT_LIST:          0 GB         2 KB      0.00%

Signed-off-by: Keith Packard <keithp@keithp.com>